Problem: Apply the distributive property to factor out the greatest common factor of all three terms. ${9a - 18b + 21c} =$
Let's find the greatest common factor of ${9}$, ${18}$, and ${21}$. ${3}$ is the greatest common factor of ${9}$, ${18}$, and ${21}$. $\phantom{=}{9}a - {18}b + {21}c$ $={3}\left(\dfrac{{9}a}{{3}}-\dfrac{{18}b}{{3}}+\dfrac{{21}c}{{3}}\right)$ $={3}\left(3a-6b+7c\right)$